I'm in a similar situation this summer and fostering a 15-year old single cat for a colleague for a while during their overseas vacation and home remodel. I already have a foster room set up to use as kitty hotel for feline guests. We'll have to see how it goes and whether she's even interested in meeting my kitty pride.

I think the best strategy if you don't know whether the cats will get along is to be prepared to keep your guest kitty separate for the entire time if necessary with occasional "outings" walking through the house while your cats are safely contained someplace else. So far I haven't really had any issues with cats not getting along though, as long as you let them take it at their own pace and provide them with a "safe room" to retreat to. With shy cats, I always wait until they are no longer hiding from me before I let them out of the foster room. Otherwise there's a high chance they might find a hiding spot elsewhere in the house and you can't get them back into their foster room. I always keep the fosters in their room for at least a week unless they are really friendly and eager to get out. Once they come out of hiding and seem curious about what's outside the room, I close the doorway off with pet gates from floor to ceiling so they can sit and watch what's going on and interact with cats and people on the other side of the gate. That works really well for me and gets even the shy cats interacting pretty quickly because the gate makes them feel safe by securing the boundaries of their new territory without having to fight for it.

Other things to make it easier for your guest: Find out or get the usually food and litter from the cat's owner so the cat doesn't have to deal with a change in litter box or food on top of being in a strange place. If the cat has a favorite bed, toy, scratch post etc, that might help, too. I also use a Feliway plugin in the "guest room" to help create a soothing atmosphere.

Another thing to keep in mind is to get vet info and an emergency contact and have the owner make arrangements with the vet if you need to bring the cat in for treatment or in the case of an emergency.

As for outdoors/indoors kitties: They'll be so busy with investigating and getting used to a new place, they won't miss the outdoors for a while. Even if you were planning to let them outside, you should keep them strictly indoors (or contained by an enclosure) for at least a couple of weeks if not months before letting them out.

Well, Jessie arrived about an hour ago. She made herself completely at home in the spare room. She looks as bright as a button. Keira (my daughter) is stunned that she hasn't gone into hiding. She's very interested in the 4ft cat tree, so much that keira asked me to order her one. Tiggz popped in to have a sniff & didn't seem to care that she was she was there, he was more interested in checking if she had any food in her cat bowl. It was a quick unplanned encounter but it's looking good
I'll try & take some photos later

Well, here's Jessie on her second night of her hols. She's such a good girl, never here a peep out of her, but boy can she molt!!!! She's in the spare room in the day time then when my other lot go to bed we let out so she can explore. She chooses to come to bed with me then when Ian comes to bed she just potters around doing her own thing, jumping on & off the bed for intermitent snuggles

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